can i fill in the number field with the difference in days between the date the task was created and

[Viktoria Soldatova]

can i fill in the number field with the difference in days between the date the task was created and the date it was solved.
using automation.

[Francis Batilo]

@Viktoria Soldatova

You can use the resolution date to indicate when your issue has been completed. For your requirement, however, the easiest way to do that is to use the diff function that is inherent on all date fields for Jira via automation. 

So, if you want to test this out, you can try creating a manual automation trigger and set it up to comment on your issue, based on the outcome that you want. In this example, I have resolution date - created date with the format of days.

resolution date: {{resolutiondate}}
created date: {{issue.created}}
{{issue.created.diff(resolutiondate).days}}

For more info, you can refer to this Cloud automation documentation: Jira smart values - date and time | Cloud automation Cloud | Atlassian Support

I hope this helps!

Cheers,

Francis